It is well known to use oil-soluble overbased metal salts of carboxylic acids as detergent additives in lubricating oils. The basicity of the salts does not only improve the detergent properties of the oils but it also provides the oils with an alkaline reserve which neutralizes any acidic compound which is formed during the operation of the engine in which the lubricating oil composition is used.
Solutions of oil-soluble overbased salts in a lubricating base oil sometimes have a tendency to gel. It is evident that this gelling tendency may lead to difficulties when such solutions are used in practice. This problem has been known in the art for a long time and in GB-A-No. 818,325 a solution is proposed. This patent specification proposes to add an oil-soluble compound which contains a polar group to the composition. Examples of such compounds are mono- or polyhydric alcohols such as methanol, hexanol and decanol, alkylamines such as decylamine, alkyl phenol, alkyl aromatic carboxylic acids and hydrocarboxylic acids, aliphatic carboxylic acid, naphthenic acids, sulphonic acids, phosphoric acids and their salts. From the Examples in this reference it is apparent that considerable amounts of these compounds are required to get the desired result, especially in the case of overbased alkaline earth metal salts. Further, the use of a carboxylic acid as a stabilizing agent, as is described in Examples of the British patent specification, reduces the overall basicity, calculated as the total equivalent of metal over the total equivalent of acid, thereby decreasing the desirable alkaline reserve. Hence such stabilizing agents are not satisfactory.
Applicants have now found that other compounds which do not necessarily have to fulfil the requirement set by GB-A-No. 818,325, i.e. that they have a polar group and an oleophilic group, stabilise lubricating oil compositions, even at low concentrations, without reducing the alkaline reserve in solutions of overbased salts.